Sandwich construction shells, in which face sheets are joined by a shear-carrying core, are the minimum weight solution for a wide range of structures. Composite materials such as graphite/epoxy may be used to make lighter structures than the metals that were formerly applied to most weight-critical structures. Joint designs have been lagging behind the development of the acreage areas of these structures.
Joining approaches that have been traditionally used for metal structures may not be applied unmodified to composite structures because of the lack of ductility in composites and limited bearing capability of thin composite face sheets. Sandwich structures may present further challenges since the face sheet-to-core bond may be compromised in combined shear and peel if subjected to concentrated loads. The state of the art attempts to avoid these problems by transitioning from sandwich construction to thick solid laminates at the edge of the shell and then using a metal joint member which can be fastened conventionally. However, the cost of mechanically attaching metal frames to solid laminate may be high due to the requirement to drill a large number of close tolerance holes in the composite.
An additional shortcoming associated with many conventional sandwich edge joints is that the joints may transition to an asymmetrical flange configuration which may cause tension loads across the joint to put the adjacent shell in bending. To carry these secondary bending loads in addition to the primary in-plane loads, it may be necessary to locally reinforce the shell. This may add additional weight that may not be required in a joint structure with straighter load paths.
Coefficient of thermal expansion mismatch between metal joint elements and adjacent composites may result in additional stresses in the structure. For structures which are exposed to wide ranges of temperatures, such as launch vehicle components, substantial weight penalties may be imposed by the need to either reinforce the structure to carry these mismatch loads or soften the structure radially to mitigate strain mismatch.
Therefore, a highly weight-efficient, combination bonded and mechanically-fastened joint configuration for composite sandwich shell edges is needed.